1. Field of the Invention
The present invention relates to a method of manufacturing a light emitting device that uses an organic compound as a light emitter, in particular, a method of manufacturing a light emitting device by using an ink jet method.
2. Description of the Related Art
Organic light emitting elements are formed so as to have a structure in which a thin film containing an organic compound in which fluorescence or phosphorescence can be obtained, is sandwiched between a pair of electrodes composed of an anode and a cathode. This light emitting mechanism is regarded as a phenomenon in which electrons injected from the cathode and holes injected from the anode recombine in a light emitting layer containing a light emitting substance, forming molecular excitons, and light is emitted when the molecular excitons return to a base state. Light emission from a singlet excitation state (fluorescence) and light emission from a triplet excitation state (phosphorescence) exist as light emitting processes. Light emission having a brightness of several thousands to several tens of thousands of cd/m2 is possible, from blue color light to red color light, by appropriately selecting organic compound materials and dopants, even with an applied voltage equal to or less than 10 V. In principle, therefore, it can be considered that it is sufficiently possible to apply this type of light emission to display devices and the like.
Consideration of both high molecular weight compounds and low molecular weight compounds for use as organic compounds is advancing, and development is progressing. However, whichever is used, they are difficult to be applied to patterning processes such as photolithography because they have low resistance to heat. Ink jet methods are being developed in order to overcome that problem, and photolithography patterning processes are unnecessary because a pattern is drawn directly on a substrate.
Regarding the ink jet methods, a technique of manufacturing an active matrix organic EL display is disclosed in JP-A-10-012377. Pixel electrodes are formed on a glass substrate having thin film transistors, and red, green, and blue color light emitting layers are formed on the pixel electrodes for each pixel by the ink jet method.
As the organic compound materials used for ink jet process, precursors of cyanopolyphenylenevinylene, polyphenylenevinylene, and the like; derivative of aromatic diamine, oxydiazole, distylarylene, triphenylamine, distyryl, and the like; and complexes such as quinolinol metal, azomethine zinc, porphyrin zinc, benzoxazole zinc and phenanethroline europium are known.
The aforementioned organic compounds, in a state in which they are dissolved or dispersed in a solvent (hereafter referred to as a “composition”) are dripped from an ink head of an ink jet printing apparatus, forming a film on a substrate. Physical properties of the composition such as viscosity, surface tension, and drying speed become vital parameters. Furthermore, the geometric structure of the ink head and the drive conditions are very important for dripping the composition onto the substrate with good reproducibility, and the weight of the composition discharged, its direction, period, and the like are parameters.
A piezoelectric element is used in discharging the composition from the ink head in the ink jet method. The volume of a container filled with the composition is changed by utilizing vibrations of the piezoelectric element, discharging the composition to the outside.
The amount of the composition discharged from the ink head one time is from 10 to 40 pl, and it can be considered that a viscosity of 1 to 20 cp is good. If the viscosity is low, then a desired film thickness cannot be obtained. Thus, problems develop, such as the composition flowing out on the surface of the substrate after impact, causing a pattern to become wider than necessary. Further, if the viscosity is too high, then problems such as the composition not being able to be discharged out smoothly from a discharge port of the ink head, one drop of the discharged compound being pulled into the shape of a thread, causing defective pattern shapes after impact.
Solvents that vaporize after being dripped onto the substrate are suitable as solvents for the composition. However, if dripping is not normally performed continuously, then the solvent will volatilize and the composition will harden near the discharge port. For example, if a very volatile solvent such as toluene is used, then it is necessary to be particularly careful. Even if discharging is performed continuously, solids gradually grow in the vicinity of the discharge port, and in the worst cases, close off the nozzle. Even if the solids in the vicinity of the discharge port to not reach such a level, they will change the direction that the composition is discharged at, causing a remarkable drop in the impact precision. In addition, a defect develops in which the amount of the composition discharged from the nozzle is reduced because the diameter of the nozzle becomes smaller, thus reducing the thickness of an organic compound layer formed on the substrate. In order to prevent these problems, it is necessary to perform frequent cleaning of the ink head in order to prevent clogging by solids with a conventional ink jet method.